The present invention relates to a process for the formation of a filter cake on a porous filter material, particularly on a filter cloth, in which with the aid of different gas pressures on either side of the filter material when the filtrate passes through the pores of the filter material, solids particles are separated from a suspension and deposited on the filter material and in which the filter cake enriched with the filtrate is at least partly dried with the aid of gas passed through the filter cake.
Such a process is known from U.S. Pat. No. 2,332,917. The known apparatus is used for arranging a relatively coarse screen in the form of a supporting fabric beneath the filter cloth, so as to limit the bulging of the latter when a compressed air shock is used for ejecting the filter cake.
There is a fundamental risk that the filter cake will crack during the drying phase, air penetrating through the cracks and cloth, so that the vacuum collapses or significantly decreases. As soon as cracks appear, the filter cake remains wet, i.e. the further suction drying of the filter cake stops immediately following the formation of the cracks. This effect is also disadvantageous if such a filter cake also has to be washed. The washing liquid is substantially only sucked through the cracks, because they take the path of least resistance. The only solution is to bring the product to the desired purity by intermediate mashing several times.
In extreme cases crack patterns form on the cake surface. Thin filter cakes have a lesser tendency to cracking than thick filter cakes of the same material. The cracks can then pass completely through the filter cake up to the filter cloth.
As the cracks are prejudicial during the washing and suction drying for the aforementioned reasons, attempts have hitherto been made to remedy this problem in the case of continuous filters (drum/belt filters), in that they are pressed together by press rolls or belts. In the case of horizontal filtration using plate or belt filters, through feeding the suspension on a number of occasions onto the already predried and therefore cracked filter cake, it is possible to make good the cracks by a filtering-in solid. However, it is possible to prove with dye tests that the porosity of the subsequently filtered-in material is higher than that of the original material. This leads to a certain inhomogeneity of the cake with the corresponding disadvantages. This possibility does not exist with disk filters, so that the cracks cannot be eliminated.
If this is known when designing the installation, no disk filters can be used and instead it is necessary to employ drum filters with the aforementioned press rolls or belts, which leads to higher and in fact roughly double the costs.
Another possibility of stopping cracks is to add small amounts of solids, e.g. pulp or glass fibers to the suspension to be filtered, but this is seldom possible for the reasons of the purity of the material or costs.